Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light or soft X-ray radiation (EUV). Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a transmissive or reflective mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
In EUV projection exposure apparatus the illumination system usually includes a mirror array (sometimes also referred to as faceted mirror) which directs the projection light produced by the EUV light source towards the system pupil surface so that a desired intensity distribution is obtained in the system pupil surface.
WO 2005/026843 A2 proposes for a DUV illumination system to use a mirror array that illuminates the pupil surface. For increasing the flexibility in producing different angular distribution in the mask plane, each of the mirrors can be tilted about two perpendicular tilt axes. A condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This known illumination system makes it possible to produce on the pupil surface a plurality of light spots, wherein each light spot is associated with one particular microscopic mirror and is freely movable across the pupil surface by tilting this mirror. It is also proposed to vary the size of the spots by using adaptive mirrors having a mirror surface whose shape can be varied to a limited extent using suitable actuators, for example piezoelectric actuators.
US 2005/0018269 A1 discloses a correction device which makes it possible to heat up certain portions of selected mirrors contained in a projection objective of a microlithographic exposure apparatus. To this end a light ray scans over the portions of the mirrors to be heated up. The device makes it possible to increase the temperature very selectively so that a desired, in particular a rotationally symmetric, temperature distribution can be achieved. In one embodiment the desired temperature distribution is determined such that the heated mirror changes its shape in a predetermined manner, thereby correcting aberrations produced in other optical elements of the objective.
WO 2004/092843 A2 discloses a correction device for a EUV projection objective of a microlithographic exposure apparatus that directs correction light to one of the large mirrors of the objective. The correction light is controlled such that the temperature in the vicinity of the reflective surface comes close to the temperature where the coefficient of thermal expansion of the mirror substrate is zero.
EP 0 532 236 A1 discloses another correction device for a EUV projection objective of a microlithographic exposure apparatus. In one embodiment infrared radiation is directed on one of the large mirrors of the objective. The infrared light is controlled such that the shape of the mirror does not substantially alter even under the impact of the high energy EUV projection light. In other embodiments heating or cooling devices are integrated into the mirror support for the same purpose.
The mirror array including adaptive mirrors as disclosed in the aforementioned WO 2005/026843 A2 is particularly advantageous because additional reflective power may be added to correct for non-ideal optical properties of a subsequent condenser, or for aberrations caused by material defects and manufacturing tolerances. However, the use of piezoelectric actuators proposed in this document has some significant drawbacks. In order to achieve a desired curvature of the mirror surface, it is desirable to provide a large number of such actuators which adds to the system complexity. For example, a very large number of electrical leads have to be provided for individually controlling the piezoelectric actuators. In a mirror array including several thousand mirror elements on a total area of less than 100 cm2 the electrical wire density becomes critical. Apart from that it is difficult to obtain the desired surface shape of the mirror elements under varying temperature conditions.